The roof ties in all the body lines that come up from the quater panel to give it that factory look. But the roof has five ribs that have to end to get that factory look. Inorder to do that the Carl Green method would be to take the last 6 inches of roof from the wagon and bring it forward and butt weld it to finish the ribs. As seen in the pictures below.

A slow and pain staking process, but well worth the time so there is little body work across the roof.